The silicon-integrated circuit has dominated electronics and has helped the industry grow to become one of the world's largest industries. However, due to a combination of physical and economic reasons, the miniaturization that has accompanied the growth of silicon integrated circuits is reaching its limit. The present scale of electronic devices is on the order of tenths of micrometers (μm). However, new solutions are being proposed to form electronic devices on an ever smaller scale, such as a nanometer (nm) scale.
Prior proposed solutions to the problem of constructing nanometer scale devices have involved (1) the utilization of extremely fine scale lithography using X-rays, electron, ions, scanning probes, or stamping to define the device components; (2) direct writing of the device components by electrons, ions, or scanning probes; or (3) the direct chemical synthesis and linking of components with covalent bonds. However, the wafer on which the devices are built must be aligned to within a fraction of a nanometer in at least two dimensions for several successive stages of lithography, followed by etching or deposition to build the devices. This level of control will be extremely expensive to implement. The second proposed solution is a serial process, and direct writing a wafer full of complex devices, each containing trillions of components, could well require many years. Finally, with regard to the third proposed solution, the only known chemical analogues of high information content circuits are proteins and DNA, both of which have extremely complex and, to date, unpredictable secondary and tertiary structures that causes them to twist into helices, fold into sheets, and form other complex 3D structures that will have a significant and usually deleterious effect on their desired electrical properties, as well as make interfacing them to the outside world impractical.
One conventional approach to nanometer-scale devices, includes use of crossed nano-scale wires that are joined at their intersecting junctions with bi-stable molecules. Wires, such as silicon, carbon and/or metal, are formed in two dimensional arrays. A bi-stable molecule, such as rotaxane, pseudo-rotaxane, or catenane, is formed at each intersection of a pair of wires. The bi-stable molecule is switchable between two states upon application of a voltage along a selected pair of wires.
One conventional method of constructing a nanometer scale transistor (a three-terminal device with gain) involves the precise positioning of three or four components within a nanometer. A quantum dot is positioned between two wires, which act as the source and drain of the transistor, in tunneling contact with the quantum dot. This is known as a single-electron transistor, or SET. A third wire is positioned in capacitive contact with the dot, which is the gate. The voltage on the gate changes the energy levels in the quantum dot, which creates a Coulomb blockade to current flowing from the source to the drain.